Critical Power Calculator
Critical power (CP) is the asymptote of the power-duration curve: the highest sustained power that does not progressively deplete a finite anaerobic reserve. That reserve, W prime, is the fixed amount of work available above CP before exhaustion. Given two maximal efforts of different durations, the relationship work equals W prime plus critical power times time is a straight line, so two work-time points determine both parameters exactly. This calculator takes two efforts (power and duration for each), computes the work done in each, and solves for CP in watts and W prime in joules and kilojoules. Use two genuinely maximal efforts of clearly different lengths for a meaningful result.
Two-test critical power formula
Work: W1 = P1 * t1, W2 = P2 * t2 (joules, with power in watts and time in seconds)
Critical power: CP = (W2 - W1) / (t2 - t1)
Anaerobic capacity: W' = W1 - CP * t1
W' in kilojoules = W' / 1000
Worked example: P1 = 320 W for 180 s gives W1 = 57,600 J; P2 = 260 W for 720 s gives W2 = 187,200 J. CP = (187,200 - 57,600) / (720 - 180) = 240.00 W and W' = 57,600 - 240 * 180 = 14,400 J (14.40 kJ).
Critical power notes
- Both efforts must be genuinely maximal for the durations chosen, or CP will be underestimated.
- The two durations should differ clearly; pairing a short and a longer effort gives a stable fit.
- CP is reported in watts and W prime in joules and kilojoules.
- CP is close to functional threshold power but is derived from a model rather than a fixed percentage.
- Re-test periodically, because CP and W prime change with training.
Critical power: frequently asked questions
What is critical power?
Critical power (CP) is the highest power output a person can theoretically sustain for a long time without continuing to draw down a finite anaerobic energy reserve. It marks the boundary between the heavy and severe exercise domains and is a key endurance benchmark in cycling and other power-measured sports.
What is W prime?
W prime, written W', is the finite amount of work that can be performed above critical power before exhaustion. It is measured in joules (or kilojoules) and represents the anaerobic work capacity, drawn down during efforts harder than CP and replenished during recovery below CP.
How are CP and W prime calculated from two tests?
Using two maximal efforts of power P1 for time t1 and P2 for time t2, total work is W = P times t. Because W = W' + CP times t, the two work-time points give CP = (W2 minus W1) divided by (t2 minus t1), and W' = W1 minus CP times t1. This is the standard two-parameter linear work-time model.
Which test durations should I use?
Use two all-out efforts of clearly different durations, commonly one around 3 minutes and one around 12 to 20 minutes, both as maximal as possible. The two points should sit on the linear part of the work-time relationship, so very short sprints below about 2 minutes are usually avoided.
Is critical power the same as FTP?
They are related but not identical. Functional threshold power (FTP) is often estimated as a fixed percentage of a 20 minute test, while critical power comes from a model fitted to two or more efforts. CP is generally close to, but not always equal to, FTP.
Official sources
- National Center for Biotechnology Information (PubMed): Critical power and the power-duration relationship literature.
- American College of Sports Medicine: Exercise physiology resources.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.